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Basic PL/SQL Syntax Elements
1. PL/SQL Block Structure
In PL/SQL, as in most other procedural languages, the smallest meaningful grouping of code is
known as a block. A block is a unit of code that provides execution and scoping boundaries for
variable declarations and exception handling. PL/SQL allows you to create anonymous blocks
(blocks of code that have no name) and named blocks , which may be procedures, functions, or
triggers.
In the sections below, we review the structure of a block and focus on the anonymous block. We'll
explore the different kinds of named blocks later in this chapter.
A PL/SQL block has up to four different sections, only one of which is mandatory. Figure
illustrates the block structure.
Figure. The PL/SQL block structure
Header
Used only for named blocks . The header determines the way the named block or program
must be called. Optional.
Declaration section
Identifies variables, cursors, and sub-blocks that are referenced in the execution and
exception sections. Optional.
Execution section
Contains statements that the PL/SQL runtime engine will execute at runtime. Mandatory.
Exception section
Handles exceptions to normal processing (warnings and error conditions). Optional.
1.2. Anonymous blocks
When someone wishes to remain anonymous, that person goes unnamed. The same is true of the
anonymous block in PL/SQL, which is shown in Figure : it lacks a header section altogether,
beginning instead with either DECLARE or BEGIN. That means that it cannot be called by any
other blockit doesn't have a handle for reference. Instead, anonymous blocks serve as containers
that execute PL/SQL statements, usually including calls to procedures and functions.
The general syntax of an anonymous PL/SQL block is as follows:
[ DECLARE
... declaration statements ...]
BEGIN
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... one or more executable statements ...
[ EXCEPTION
... exception handler statements ...]
END;
Figure. An anonymous block without declaration and exception sections
The square brackets indicate an optional part of the syntax. You must have BEGIN and END
statements, and you must have at least one executable statement. Here are a few examples:

A bare minimum anonymous block:

BEGIN

DBMS_OUTPUT.PUT_LINE(SYSDATE);
END;






A functionally similar block, adding a declaration section:
DECLARE
l_right_now VARCHAR2(9);
BEGIN
l_right_now := SYSDATE;
DBMS_OUTPUT.PUT_LINE (l_right_now);
END;











The same block, but including an exception handler:
DECLARE
l_right_now VARCHAR2(9);
BEGIN
l_right_now := SYSDATE;
DBMS_OUTPUT.PUT_LINE (l_right_now);
EXCEPTION
WHEN VALUE_ERROR
THEN
DBMS_OUTPUT.PUT_LINE('I bet l_right_now is too small '
|| 'for the default date format!')
END;
2. The PL/SQL Character Set
A PL/SQL program consists of a sequence of statements, each made up of one or more lines of text.
The precise characters available to you will depend on what database character set you're using. For
example , Table illustrates the available characters in the US7ASCII character set .
Table. Characters available to PL/SQL in the US7ASCII character set
Type
Characters
Letters
A-Z, a-z
Digits
09
3
Table. Characters available to PL/SQL in the US7ASCII character set
Type
Characters
Symbols
~!@#$%*()_-+=|:;"'<>,.?/^
Whitespace
Tab, space, newline, carriage return
Every keyword, operator, and token in PL/SQL is made from various combinations of characters in
this character set. Now you just have to figure out how to put them all together!
Keep in mind that PL/SQL is a case-insensitive language. That is, it doesn't matter how you type
keywords and identifiers; uppercase letters are treated the same way as lowercase letters unless
surrounded by delimiters that make them a literal string. By convention, the authors of this book
prefer uppercase for built-in language keywords and lowercase for programmer-defined identifiers.
A number of these charactersboth singly and in combination with other charactershave a special
significance in PL/SQL. Table lists these special symbols.
Table. Simple and compound symbols in PL/SQL
Symbol
Description
;
Semicolon: terminates declarations and statements
%
Percent sign: attribute indicator (cursor attributes like %ISOPEN and indirect
declaration attributes like %ROWTYPE); also used as multi-byte wildcard symbol
with the LIKE condition
_
Single underscore: single-character wildcard symbol in LIKE condition
@
At-sign: remote location indicator
:
Colon: host variable indicator, such as :block.item in Oracle Forms
**
Double asterisk: exponentiation operator
< > or != or
Ways to denote the "not equal" relational operator
^= or ~=
||
Double vertical bar: concatenation operator
<< and >>
Label delimiters
<= and >=
Less than or equal to, greater than or equal to relational operators
:=
Assignment operator
=>
Association operator for positional notation
..
Double dot: range operator
--
Double dash: single-line comment indicator
/* and */
Beginning and ending multi-line comment block delimiters
Characters are grouped together into lexical units, also called atomics of the language because they
are the smallest individual components. A lexical unit in PL/SQL is any of the following: identifier,
literal, delimiter, or comment. These are described in the following sections.
3. Identifiers
An identifier is a name for a PL/SQL object, such as a variable, program name, or reserved word.
The default properties of PL/SQL identifiers are summarized below:

Up to 30 characters in length
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


Must start with a letter
Can include $ (dollar sign), _ (underscore), and # (pound sign)
Cannot contain any "whitespace " characters
If the only difference between two identifiers is the case of one or more letters, PL/SQL treats those
two identifiers as the same. For example, the following identifiers are all considered by PL/SQL to
be the same:
lots_of_$MONEY$ LOTS_of_$MONEY$ Lots_of_$Money$
3.1. NULLs
The absence of a value is represented in Oracle by the keyword NULL. As shown in the previous
section, variables of almost all PL/SQL datatypes can exist in a null state (the exception to this rule
is any associative array type, instances of which are never null). Although it can be challenging for
a programmer to handle NULL variables properly regardless of their datatype, strings that are null
require special consideration.
In Oracle SQL and PL/SQL, a null string is usually indistinguishable from a literal of zero
characters, represented literally as '' (two consecutive single quotes with no characters between
them). For example, the following expression will evaluate to TRUE in both SQL and PL/SQL:
'' IS NULL
While NULL tends to behave as if its default datatype is VARCHAR2, Oracle will try to implicitly
cast NULL to whatever type is needed for the current operation. Occasionally, you may need to
make the cast explicit, using syntax such as TO_NUMBER(NULL) or CAST(NULL AS
NUMBER).
3.2. Literals
A literal is a value that is not represented by an identifier; it is simply a value.
A string literal is text surrounded by single quote characters, such as:
'What a great language!'
Unlike identifiers, string literals in PL/SQL are case-sensitive. As you should expect, the following
two literals are different.
'Steven'
'steven'
So the following condition evaluates to FALSE:
IF 'Steven' = 'steven'
3.3. Numeric literals
Numeric literals can be integers or real numbers (a number that contains a fractional component).
Note that PL/SQL considers the number 154.00 to be a real number of type NUMBER, even though
the fractional component is zero and the number is actually an integer. Internally, integers and reals
have a different representation, and there is some small overhead involved in converting between
the two.
You can also use scientific notation to specify a numeric literal. Use the letter "E" (upper- or
lowercase) to multiply a number by 10 to the nth powerfor example, 3.05E19, 12e-5.
Beginning in Oracle Database 10g Release 1, a real can be either an Oracle NUMBER type or an
IEEE 754 standard floating-point type . Floating-point literals are either binary (32-bit) (designated
with a trailing F) or binary double (64-bit) (designated with a trailing D).
In certain expressions, you may use the named constants summarized in Table as prescribed by the
Institute of Electrical and Electronics Engineers (IEEE) standard.
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Table. Named constants for BINARY_FLOAT and BINARY_DOUBLE
Description
BINARY_FLOAT (32-bit)
BINARY_DOUBLE (64-bit)
"Not
a
number"
(NaN); result
BINARY_FLOAT_NAN
of divide by 0
or
invalid
operation
BINARY_DOUBLE_NAN
Positive
infinity
BINARY_DOUBLE_INFINITY
BINARY_FLOAT_INFINITY
Maximum
finite number
that is less
BINARY_FLOAT_MAX_NORMAL
than
the
overflow
threshold
BINARY_DOUBLE_MAX_NORMAL
Smallest
normal
number;
underflow
threshold
BINARY_DOUBLE_MIN_NORMAL
BINARY_FLOAT_MIN_NORMAL
Maximum
positive
number that BINARY_FLOAT_MAX_SUBNORM BINARY_DOUBLE_MAX_SUBNORMA
is less than AL
L
the underflow
threshold
Absolute
minimum
BINARY_FLOAT_MIN_SUBNORM
number that
AL
can
be
represented
BINARY_DOUBLE_MIN_SUBNORMA
L
3.4. Boolean literals
PL/SQL provides two literals to represent Boolean values: TRUE and FALSE. These values are not
strings; you should not put quotes around them. Use Boolean literals to assign values to Boolean
variables, as in:
DECLARE
enough_money BOOLEAN; -- Declare a Boolean variable
BEGIN
enough_money := FALSE; -- Assign it a value
END;
On the other hand, you do not need to refer to the literal value when checking the value of a
Boolean expression. Instead, just let that expression speak for itself, as shown in the conditional
clause of the following IF statement:
DECLARE
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enough_money BOOLEAN;
BEGIN
IF enough_money
THEN
...
A Boolean expression, variable, or constant may also evaluate to NULL, which is neither TRUE
nor FALSE.
3.5. The Semicolon Delimiter
A PL/SQL program is made up of a series of declarations and statements. These are defined
logically, as opposed to physically. In other words, they are not terminated with the physical end of
a line of code; instead, they are terminated with a semicolon (;). In fact, a single statement is often
spread over several lines to make it more readable. The following IF statement takes up four lines
and is indented to reinforce the logic behind the statement:
IF salary < min_salary (2003)
THEN
salary := salary + salary * .25;
END IF;
There are two semicolons in this IF statement. The first semicolon indicates the end of the single
executable statement within the IF-END IF construct. The second semicolon terminates the IF
statement itself.
3.6. Comments
Inline documentation, otherwise known as comments , is an important element of a good program.
While there are many ways to make your PL/SQL program self-documenting through good naming
practices and modularization, such techniques are seldom enough by themselves to communicate a
thorough understanding of a complex program.
PL/SQL offers two different styles for comments : single and multi-line block comments.
The single-line comment is initiated with two hyphens ( -- ), which cannot be separated by a space
or any other characters. All text after the double hyphen to the end of the physical line is considered
commentary and is ignored by the compiler. If the double hyphen appears at the beginning of the
line, the whole line is a comment.
In the following IF statement, I use a single-line comment to clarify the logic of the Boolean
expression:
IF salary < min_salary (2003) -- Function returns min salary for year.
THEN
salary := salary + salary*.25;
END IF;
While single-line comments are useful for documenting brief bits of code or ignoring a line that you
do not want executed at the moment, the multi-line comment is superior for including longer blocks
of commentary.
Multiline comments start with a slash-asterisk (/*) and end with an asterisk-slash (*/). PL/SQL
considers all characters found between these two sequences of symbols to be part of the comment,
and they are ignored by the compiler.
The following example of multi-line comments shows a header section for a procedure. I use the
vertical bars in the left margin so that, as the eye moves down the left edge of the program, it can
easily pick out the chunks of comments:
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PROCEDURE calc_revenue (company_id IN NUMBER)
/*
| Program: calc_revenue
| Author: Steven Feuerstein
*/
IS